1. Field of the Invention.
This invention is concerned with methods for the purification of inorganic salts in general and more particularly with the purification of metal halides.
2. Description of the Prior Art.
Metal halides are well-known in the art. However, comparatively recent developments in the field of laser technology has resulted in a demand for metal halides that are extremely pure. While powdered KCl 99.999% pure in the metal ion is easily available, the anion purity may not be any better than 99.9% pure.
Metal halide crystals grown from state-of-the-art purified starting materials contain trace cation and anion contaminants which when subject to high energy laser applications, cause undesirable optical absorption and structural failures. Anion purity is a prime concern for high-power IR window material, because anions, particularly polyatomic ones like OH.sup.- , contribute significantly to IR absorption through the fundamentals and combinations of their vibrational motions. The usual metal ion impurities mostly affect transmission in the visible or ultraviolet. Bivalent metal ion doping has been employed to increase the crystal's mechanical strength. However, this process exacts a tradeoff in the IR transmission, possibly due to the ability of bivalent cation impurities to stabilize the hydroxide ion (OH.sup.- ) and cation vacancies in alkali metal halides. Also, because of the extremely limited solubility of such dopants in the pure solid, the achievement of bulk homogeneity over a large piece presents a severe constraint.
The patent literature (J. O. Hay, U.S. Pat. No. 2,640,755) discloses an alkali metal halide purification method based on the addition of halide reactants to the metal halide desired to be purified in solution to form compounds with the impurities which can be precipitated from the solution and effectively separated from the target compound. This process is a multistep one and limited in effectivity in that it only tends to remove metal ion impurities.
My invention is considerably more efficient in terms of steps required to complete the process and in terms of the relative purity of the resultant product. It is possible, by utilizing my invention, to proceed from the purification of metal halide powders to the growth of optically pure metal halide crystals in succession with no intervening handling of the material.